1. Field of the Invention
The present invention relates to an projection imaging type electron microscope which observes or inspects object surfaces by illuminating sample surfaces with an electron beam and using the secondary electrons, reflected electrons, or the like that are generated as a result.
2. Description of the Related Art
Mapping type electron microscopes are electron microscopes in which sample surfaces are observed in two dimensions by using an electron beam optical system to illuminate the sample surface with an electron beam, and using an electron beam optical system to focus an image of the secondary or reflected electrons generated as a result on the detection surface of a detector. Unlike an SEM, such an electron microscope makes it possible to reduce the number of times that scanning is performed; accordingly, the required sample observation time can be shortened, so that this type of microscope has attracted attention as an inspection device for micro-devices such as semiconductors.
An example of a microscope that is conceivable as such an projection imaging type electron microscope is shown in FIG. 5. An illuminating beam 34 emitted from a cathode 31 passes through a Wehnelt electrode 44, a first anode 45, a second anode 46 and an illumination electron optical system 32, and is incident on an electromagnetic prism 33. After the optical path of the illuminating beam 34 is altered by the electromagnetic prism (E×B) 33, the beam 34 passes through an objective electron optical system 37, and illuminates the surface of a sample 36.
When the illuminating beam 34 is incident on the sample 36, secondary electrons, backscattered electrons and reflected electrons with a distribution corresponding to the surface shape, distribution of materials, variation in potential and the like (referred to collectively as generated electrons 38) are generated from the sample 36. These generated electrons 38 pass through the objective electron optical system 37, electromagnetic prism 33 and image focusing electron optical system 39, and are projected onto an MCP (micro channel plate) detector 40; accordingly, an image is formed on the CCD camera 43 via an optical image projection optical system 42. Furthermore, 35 indicates a sample stage.
The optical path of the electron beam in the essential parts of such an projection imaging type microscope is shown in FIG. 6. In FIG. 6, the objective electron optical system 37 is shown as consisting of lenses 37a and 37b, and an aperture diaphragm 37c, and the image focusing electron optical system 39 is shown as consisting of lenses 39a, 39b, 39c and 39d. 
After being caused to converge by the action of the lens 37a, the illuminating beam 34 diverges, and is caused to illuminate the sample 36 in a perpendicular (telecentric) manner by the action of the lens 37b. The position where the illuminating beam converges is the first crossover position in the projection image focusing optical system. The generated electrons 38 that are generated from a point on the optical axis of the sample 36 converge at the position of the electromagnetic prism 33 after leaving the objective electron optical system 37; subsequently, these electrons are acted upon by the lenses 39a, 39b, 39c and 39d, and are focused as an image on the MCP detector.
The generated electrons 38b (principal rays) that are emitted in a direction parallel to the optical axis from points that are off the optical axis of the sample 36 pass through the first crossover position, and are acted upon by the lenses 39a, 39b, 39c and 39d so that these electrons cross the optical axis twice as shown in the figures. The position where these electrons initially cross the optical axis following the first crossover position is called the second crossover position. Since the aperture diaphragm 37c is disposed in the first crossover position, the generated electrons 38 constitute the principal rays, and the opening angle of the projection electron optical system including the objective electron optical system and image focusing electron optical system is determined by the aperture diaphragm 37c. 
Furthermore, the illuminating beam 34 is arranged so that this beam illuminates the sample 36 by Koehler illumination. Specifically, the system is arranged so that the aperture diaphragm (not shown in the figures) disposed in the illumination electron optical system 32 in FIG. 5 is in a conjugate relationship with the first crossover position. The aperture diaphragm 37c is disposed in order to determine the opening angle of the projection electron optical system, and may be omitted as far as the inherent illumination electron optical system is concerned.
Furthermore, the lenses 39a and 39b act as zoom lenses, and the image focusing magnification can be varied by altering the applied voltage so that the power balance of the lenses 39a and 39b is altered. Moreover, the lenses 39c and 39d act as projection lenses.